The invention relates to a screw actuator comprising a rotatable input member which cooperates with a primary, non-rotatable output member through a ball screw coupling such that rotational movement of the input member causes axial movement of the primary output member. In particular, but not exclusively, the invention relates to a screw actuator suitable for use in controlling the angle of inclination of a stabiliser on an aircraft.
Known aircraft stabilisation systems include horizontal and vertical stabilisers mounted upon the aircraft fuselage which are arranged to provide directional stability when the aircraft is in flight. Referring to FIG. 1, the angle of inclination of the horizontal stabiliser (not shown) may be varied by means of a ball screw actuator, referred to generally as 10, comprising a rotatable input shaft 12 carrying a primary, non-rotatably mounted output nut 14. The primary nut 14 is provided with a helical groove which defines a screw thread formation 16 within which a plurality of balls 18 are in rolling engagement to provide a ball screw coupling between the input shaft 12 and the primary nut 14. Upon rotational movement of the input shaft 12, the primary nut 14 is caused to move axially relative to the input shaft 12. The primary nut 14 is coupled to the horizontal stabiliser through a suitable linkage to permit the angle of inclination of the horizontal stabiliser to be varied upon axial movement of the primary nut 14 relative to the input shaft 12.
The primary nut 14 may be coupled to a secondary nut 20 in screw threaded engagement with the input shaft 12 through a second screw thread formation 22 provided on the input shaft 12 such that axial movement of the primary nut 14 also causes the secondary nut 20 to move axially relative to the input shaft 12.
The ball screw coupling 16, 18 provides a relatively low friction coupling between the input shaft 12 and the primary nut 14, whereas the plain screw thread coupling between the input shaft 12 and the secondary nut 20 provides a relatively high friction coupling. Thus, in normal use, substantially all of the angular load on the input shaft 12 is imparted to the primary nut 14 through the low friction ball screw coupling 16, 18. The axial position of the secondary nut 20 relative to the screw thread 22 on the input shaft 12 is accurately set to ensure there is limited frictional loading of the secondary nut 20 in such circumstances.
In the event that the ball screw coupling 16, 18 fails, it is important that the actuator is stalled to prevent unwanted movement of the horizontal stabiliser. Should the primary nut 14 fail, the angular load applied by the input shaft 12 will be transferred to the secondary nut 20 through the high friction screw thread coupling such that further rotation of the input shaft 30 should cause the actuator to stall. However, it has been found that, in certain circumstances, the actuator only stalls through loading of the secondary nut 20 at a higher input load than desired. The screw thread 22 at the secondary nut 20 can therefore be caused to wear, and eventually may detach from the input shaft 12 altogether. In such circumstances, if the secondary actuator does not stall complete failure of the stabiliser may result.
It is an object of the present invention to provide a screw actuator which overcomes this problem.
According to the present invention, there is provided a screw actuator comprising a rotatable input member and a non-rotatable primary output member, wherein the input member is provided with a screw thread formation within which a plurality of spherical members are received to provide a ball screw coupling between the input member and the primary output member, a secondary output arrangement including first and second nuts in screw threaded engagement with the input member to provide a screw thread coupling between the input member and the secondary output arrangement, the first and second nuts being axially movable relative to one another, whereby, in the event that the ball screw coupling fails, relative axial movement of the first and second nuts serves to lock the first and second nuts onto the input member, thereby to stall the actuator.
In the event that the ball screw coupling fails, for example if the balls of the ball screw coupling break or the screw thread becomes stripped, load applied by the input member is transferred from the ball screw coupling to the plain screw thread coupling. Loading of the secondary output arrangement through the screw thread coupling urges the first and second nuts apart and causes the first and second nuts to be locked onto the input member, thereby stalling the actuator. The invention therefore provides the advantage that the actuator will always stall should the ball screw coupling fail.
In a preferred embodiment, the secondary output arrangement includes a ball/ramp arrangement arranged to impart relative axial movement to the first and second nuts of the secondary output arrangement in the event that the ball screw coupling fails.
The ball/ramp arrangement preferably comprises a plurality of spherical drive members which, in normal use, are urged into an equilibrium position in which they are engaged within respective recesses defined by the first and second nuts.
Preferably, the secondary output arrangement may be coupled to the primary output member.
The actuator preferably comprises a biasing arrangement, preferably comprising at least one spring, the biasing arrangement being arranged to apply a biasing force to the first nut so as to urge the spherical drive members into their equilibrium positions.
The spherical drive members are arranged to ride out of engagement with their respective recesses in the event that the ball screw coupling fails and load applied by the input member is transferred from the ball screw coupling to the screw thread coupling. As the spherical drive members ride out of engagement with their respective recesses, the first and second nuts are urged apart, against the biasing force of the biasing means, thereby causing the first and second nuts to be locked onto the input member. This ensures the actuator will always be stalled in the event that the ball screw coupling fails.
The ball screw actuator is particularly suitable for use in controlling a horizontal stabiliser on an aircraft but may also be used in other applications in which it is desirable to stall the actuator should the ball screw coupling fail.
The biasing means conveniently take the form of a multi spring assembly which acts on the first nut.
The secondary output arrangement preferably includes a non-rotatable nut housing which is axially moveable with the primary output member upon rotation of the input member, the first nut being coupled to the nut housing such that relative angular movement between the first nut and the nut housing is substantially prevented.
In a further embodiment, the secondary output arrangement includes a roller arrangement arranged to impart relative axial movement to the first and second nuts in the event that the ball screw coupling fails.
Preferably, the roller arrangement includes a threaded roller having a helix angle of substantially zero which co-operates with respective threads on radially outer surfaces on the first and second nuts.
More preferably, the threads on the radially outer surfaces of the first and second nuts are oppositely directed screw threads.
Preferably, the roller arrangement includes a shearable member coupling the second nut to the housing and arranged to shear, to break said coupling, in the event that the ball screw coupling fails, thereby to permit relative axial movement of the first and second nuts. More preferably, the shearable member is in the form of a pin.